/Engine/Source/Runtime/MeshDescription/Public/MeshDescriptionBase.h
Set the vertex instance at the given index around the polygon to the new value
void SetPolygonVertexInstances ( [FPolygonID](API\Runtime\MeshDescription\FPolygonID) PolygonID, const [TArray](API\Runtime\Core\Containers\TArray)< [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) > & VertexInstanceIDs )